This application generally relates to power management in portable computers, and more specifically to a computers with power-conserving display systems.
A portable computer is typically a self contained unit which may operate under battery power, and may be called by any number of names, such as laptop, notebook, palmtop, etc. One of the most important criteria for a portable computer user is operational time under battery power. Time management is critically important, since when the battery of a battery-powered computer is exhausted, the user is essentially xe2x80x9cstranded.xe2x80x9d Thus, users place great emphasis on the operating lifetime of portable computers, and many strategies have been used to extend the operating lifetime which can be achieved for a given amount of battery energy.
A common element of many strategies is to selectively power down certain elements of the system when the system is idle. Therefore, many smart systems have been used to detect various conditions which would indicate a standby mode, and to initiate power down or reduced power modes of operation for different elements. See generally the Advanced Power Management (APM) specification, which is hereby incorporated by reference.
Many functional techniques have been applied to reduce the power consumption of various components of the portable computer; the central processing unit can be slowed or stopped, or peripherals may be turned off when not used. However, to date the only techniques used to decrease the power required by the video subsection have involved turning off functions; for the LCD panel, such techniques include screen blanking (turning off the panel entirely after a measured inactivity of the CPU) or decreasing the overall screen brightness (dimming the screen when under battery power). Each of these techniques produce non-functional or ergonomically compromised operation.
The LCD screen is unique among the different components of the portable system in that it is the only component that normally stays active whenever the system is being used (unless a secondary display is used); unlike components such as hard disk drives, it cannot be powered down momentarily and then powered back up when the periodic need arises, while allowing the user to continue to work at the same time. Therefore, the LCD screen normally has only two distinct states when the system as a whole is powered on. A first active state, which has a full refresh rate, is used under conditions when there is abundant power or when the application requires a high refresh rate, such as in graphics animation. A second state, which is a full suspend mode in which the screen state is frozen and the screen is powered down and any lighting removed, is used to preserve battery power when the screen is not needed. Therefore, the user is left with a choice of either using the display at full power or not using the system at all.
The CRT was the original display for desktop computers, and is still the most popular by far. CRTs require a high refresh rate because the phosphor used for their displays generally has a short persistence; slow refresh rates result in dim and/or flickering displays for a CRT. However, the persistence of the available LCD display today is extremely long, over 20 milliseconds. The transition from CRTs to LCDs as a display technology carried over the fast (56-75 Hz) refresh rates of the CRT, with the result that the LCD is xe2x80x9cover refreshedxe2x80x9d when used alone (not simultaneously with a CRT). A great deal of power (typically about 10%) of the entire portable computer""s power budget today goes to the LCD panel logic only.
One strategy for power management which has been followed in the past is to power down the LCD display while freezing its state. The LCD display is inherently fairly power efficientxe2x80x94much more so than a CRT displayxe2x80x94but nevertheless does consume a significant amount of power if the backlit illumination is turned on, and also consumes an amount of power which is dependent upon the frequency of refresh.
Since the advent of super VGA standards, a substantial amount of progress has been made in providing at least the option of displays with a higher refresh rate for a given resolution. Thus, many graphics controllers permit the refresh rate to be specified as well as the parameters.
The Multimedia and motion-video applications targeted by today""s notebook computers require tremendous amounts of Video graphics controller performance. One way to get this performance is to increase the operational frequency of the Video graphics controller engine and memory interface. However, the high performance settings required for multimedia applications compromise the power requirement for the notebook computer when running xe2x80x9cbusinessxe2x80x9d applications (word processing or spreadsheet usage); the higher clock rates and greater Video section transistor count (2 megabytes of DRAM now allotted for the video only) mean greater power consumption and shorter battery life.
CRTs require a fast, xe2x80x9cnormalxe2x80x9d refresh rate of 56-75 Hz minimum, and today""s generation of graphics controllers support only xe2x80x9csynchronous dualxe2x80x9d output to both the CRT and LCD; this means that both the CRT and the LCD must share the same refresh rate and video timings. If the internal LCD is a 640-480 resolution, it will only simultaneously display to a CRT if the CRT is also displaying 640-480 resolution and at the same refresh rate. In the present generation of video controllers, the CRT and LCD are clocked at the same rates. Because of this, typical notebook PCs today always operate at xe2x80x9cfastxe2x80x9d refresh rates, and a change in refresh rate for any reason is only incidental.
The present application discloses a method and system for reducing the power consumption of an LCD display. The first element of this innovative approach to video power management decreases the operational power of the LCD panel itself without compromising screen brightness or screen function. The principle of this is to detect when the LCD only is being displayed (an external CRT is not being used) and to lower the refresh rate of the internal panel to save power.
A second element of the power conservation is a method for detecting whether a power conservation mode is enabled, and slowing down the Video Graphics Controller (xe2x80x9cVGCxe2x80x9d) engine when high performance is not required. This innovative method recognizes that video performance need not exceed the application requirements. For example, playing an MPEG movie requires high performance video, but word processing, electronic mail, or spreadsheet use does not. A method for automatically configuring appropriate refresh rates for the VGC according to system and application requirements can result in significant reductions in power consumption. The conservation method may be enabled or disabled depending on whether a CRT is active, on whether any Bus mastering video device is active, and on the amount of CPU activity to change the refresh rate and performance clocking of the video subsection.
An innovative feature of this technique is the ability to sense whether an external CRT is being used, and to switch from xe2x80x9cslowxe2x80x9d to xe2x80x9cnormalxe2x80x9d refresh rates accordingly. xe2x80x9cSlowxe2x80x9d refresh rates may be less than 50 Hertz, xe2x80x9cnormalxe2x80x9d refresh rates are typically from 56 to 75 Hz, or higher. xe2x80x9cSlowxe2x80x9d refresh is usually performed by lowering the Pixel clock rate via registers in the VGC. Accompanying the slower pixel clock may be a reduction in the panel voltage itself; this method allows the voltage of the panel to be lowered when slower refresh rates are permitted. Halving the refresh rate saves approximately half of the display power consumption directly. This invention changes refresh to slow rates (less than 50 Hz and generally 30-40 Hz) when the output is only being sent to the LCD display.
Another important feature of the disclosed inventions is they take advantage of capabilities that are already present in typical computer systems. Today""s portable computer systems generally include some sort of power management features, and most modern video controllers allow several different refresh rates to be used. This application provides an innovative method and system to take advantage of these capabilities, to provide for greatly reduced power consumption when high performance video is not required.